In a related-art image forming apparatus for use in product printing, etc., a predetermined optical output is obtained from a light source such as an LD (laser diode), etc., to expose a photosensitive body therewith and express a density of an image.
Now, it is known that, in the related art, a light emission delay time which depends on a response characteristic of a light source before obtaining a predetermined optical output from the light source occurs. Moreover, in the related art, it is known, for example, that, from a time at which a drive current is supplied to a light source to a time at which an optical output is detected, a light emission delay time occurs which depends on a parasitic capacitance of a circuit, etc., in which the light source is mounted.
Therefore, in the related-art image forming apparatus, when a time to cause an optical output is set to a short time of less than or equal to a few ns, for example, the optical output becomes less than a predetermined light amount, and the density of an image decreases, possibly causing unevenness in the image.
Thus, in the related art, schemes are provided to solve the above-described problems. For example, Patent document 1 discloses providing a charge and discharge circuit, wherein an overshoot current is generated by discharging at a time of a rise of an output of the LD (laser diode) to reduce a light emission delay time which depends on a response characteristic of a light source. Moreover, Patent document 2 discloses initially superposing a threshold current at a start time of lighting the LD and controlling a light emission amount thereof.
However, the overshoot current in Patent document 1 is generated primarily for reducing a delay time which depends on the response characteristic of the light source, so that it is difficult to improve the light emission delay time which depends on a parasitic capacitance. Moreover, in Patent document 2, while the threshold current is initially superimposed at the start time of lighting the LD, the threshold current is insufficient for charging a parasitic capacitance, so that it is difficult to sufficiently reduce a light emission delay time which depends on the parasitic capacitance. In particular, it becomes more difficult to reduce a delay time which depends on the parasitic capacitance in a circuit with a large parasitic capacitance and a light source with a large differential resistance.
A related-art semiconductor laser drive circuit is broadly classified into a non-bias technique and a bias technique.
In the non-bias technique, a bias current of a semiconductor laser is set to 0 and the semiconductor laser is driven by a pulse current which corresponds to an input signal.
Here, when the semiconductor laser with a large threshold current is driven by the non-bias technique, it requires some time before a carrier having a density which allows laser light emission is generated even when a drive current which corresponds to an input signal is applied to the semiconductor laser, leading to a light emission delay. The light emission delay does not become problematic when an input signal is sufficiently longer than a light emission delay time (when a light emission delay amount is negligible).
However, when it is necessary to drive the semiconductor laser at high speed as the laser printer, the optical disk apparatus, the digital copier, etc., increases in speed, only a pulse with a pulse width smaller than a desired pulse width may be obtained with the non-bias technique.
In order to solve the problems in the non-bias technique as described above, the bias technique is being proposed.
In the bias technique, the bias current of the semiconductor laser is set to a threshold current of the semiconductor laser and a pulse current which corresponds to the input signal is added to the bias current while applying the bias current continuously to drive the semiconductor laser.
For the bias technique, a current corresponding to a light emission threshold (a light emission threshold current) is applied to the semiconductor laser in advance, eliminating the light emission delay time.
However, for the bias technique, electricity is being turned on continuously around the light emission threshold even when the semiconductor laser does not emit light (normally 200 μWs to 300 μWs). Therefore, for optical communications using the semiconductor laser which is driven by the bias technique, an extinction ratio becomes small. The small extinction ratio of the semiconductor laser causes surface staining of an image in the laser printer, the optical disk apparatus, digital copier, etc., that uses the semiconductor laser for the light source.
In the field of optical communications, in order to solve the above-described problems, a configuration is being proposed of basically using a non-bias technique and applying a light emission threshold current immediately before causing light to be emitted (see Patent documents 3 and 4, for example).
However, recently, image forming apparatuses which use a 650 nm red semiconductor laser, a 400 nm ultraviolet semiconductor laser, etc., in the quest for an increased resolution in the laser printer, the optical disk apparatus, the digital copier, etc., are being put to practical use.
Moreover, for an increased speed in processing and an increased resolution of images, semiconductor lasers such as a VCSEL (vertical cavity surface emitting laser) in which it is easy to integrate multiple light sources are also being put to practical use.
These semiconductor lasers have a characteristic that more time, relative to the related-art 1.3 μm-band, 1.5 μm-band, and 780 μm-band semiconductor lasers, is required before a carrier having a density which allows laser light emission is generated due to reasons such as a large differential resistance thereof.
Moreover, these semiconductor lasers are able to yield only a pulse width which is smaller than a desired pulse width even with the bias technique. Therefore, a semiconductor laser driving method in the light of these characteristics is needed.
Furthermore, in a case of seeking to cause a low density to be manifested by an optical output of a short time (for example, less than or equal to several ns), a light emission output does not reach a peak strength of a beam spot. Therefore, there is a problem that, in the above-described case, the density becomes unnecessarily low, so as not to be able to cause the density to be manifested correctly.
There is also known a technique of superimposing a differential pulse at a time of a rise of a laser drive signal applied to the semiconductor laser in order to solve the above-described problem (see Patent document 5, for example).
However, with this method, a peak of the differential pulse cannot be controlled, so there is a high risk of destroying the semiconductor laser. Moreover, the time in which the differential pulse is superimposed also depends on a differential waveform. Thus, with this method, there is a problem that it is not necessarily the case that a subsequent tone manifestation increases linearly even when an initial ultra low density may be corrected for.
A technique is being proposed of providing a high-speed and high-accuracy semiconductor laser drive control and conducting a correction using four currents of a bias current; a light emission threshold current; a light emission current; and a drive auxiliary current (see Patent document 2, for example).
With the technique proposed in Patent document 2, an ideal shape of an almost square wave as an optical waveform may be obtained without question.
However, with the technique proposed in Patent document 2, a waveform of a pulse of an output signal may become narrower than a waveform of a pulse of an input signal depending on set values of the bias current and the light emission threshold current, or, in other words, a pulse narrowing phenomenon may occur.
Now, as the semiconductor lasers for use in the image forming apparatus, etc., a semiconductor laser array, the VCSEL, etc., are often used. The semiconductor lasers have various characteristics depending on the structure, wavelength characteristics, output characteristics, etc.
For example, the 650 nm-band red semiconductor laser generally has a differential resistance which is larger than that of the 780 nm-band infrared semiconductor laser. Therefore, with the red 650 nm-band red semiconductor laser, a square wave may not be obtained at high speed, so that waveform dullness may occur depending on a configuration of a driving circuit, substrate, etc.
Moreover, even with a semiconductor laser which emits an infrared light, the VCSEL, for example, has a differential resistance of a few hundreds of Os, which is very large relative to that of an edge type laser due to differences in structure. Therefore, using the VCSEL results in a CR time constant by a terminal capacitance of the VCSEL itself; a parasitic capacitance of a substrate; and a terminal capacitance of a driver. In other words, the VCSEL itself may not yield a predetermined response waveform even when mounted to a substrate even though it has a device characteristic of being able to modulate at high speed or a cutoff frequency of Ft.
Furthermore, with the semiconductor laser, there is a large fluctuation in a light emission strength relative to a current amount between an LED (light emitting diode) region up to the threshold current and an LD region on and above the threshold current. Here, when driving the image forming apparatus by increasing a current to a light emission strength from a state in which the bias current of less than or equal to the threshold current is applied, the light emission strength in the LED region is low. In other words, this case causes a light emission delay relative to a drive signal.